The shortage and increasing cost of petroleum products has stimulated considerable research and development work to reduce the weight of automobile vehicles in order to increase efficiency and gasoline mileage. One such technique under investigation is the use of a thinner gauge, higher strength steel for fabrication of vehicle structural components, such as bumper face bars, wheel components and structural brackets such as engine mounts and the like in place of conventional structural steels employed requiring thicker gauges for achieving the same strength of the resultant vehicle component. Various high strength low alloy steels of a minimum yield strength of about 80,000 pounds per square inch (550 MPa) are known which incorporate elements such as columbium, vanadium, or titanium as secondary hardening addition agents. In spite of the weight saving advantages afforded by such high strength low alloy steels, a widespread adoption thereof on a commerical scale has been inhibited due to the necessity or redesigning the specific components and providing new tooling for their fabrication due to the reduced formability of such steels due to their higher strength and resistance to deformation and elongation.
In order to overcome such problems, it has heretofore been suggested to subject certain ones of such high strength low alloy steels in an as-rolled condition to a post heat treatment to effect a conversion thereof to a two-phase microstructure and in which transformed and annealed condition, the heat treated steel is of a lower initial yield strength, facilitating its formability and deformation during fabrication into automobile components. The work hardening to which the steel is subjected during such fabrication operations causes an increase in its yield strength to a magnitude generally equal to that of its original as-rolled condition. While such a post heat treatment of high strength low alloy steels to produce a formable two-phase steel strip overcomes many of the problems associated with the formation and fabrication of lightweight, high strength automobile components, the high cost and complexity of such post heat treatment steps has detracted from a more widespread adoption of such heat treated steels. Moreover, the post heat treatment cycle requires special facilities, requiring a substantial investment or capital expenditure in order to practice the process, further detracting from a widespread commercial acceptance thereof.
The problems and disadvantages associated with the foregoing post heat treatment process are overcome in accordance with the improved high strength low alloy steel of the present invention and its method of manufacture, whereby the resultant steel strip is produced in an as-rolled condition possessed of a dual-phase microstructure, obviating the need for subjecting the steel strip product to a post heat treatment cycle, thereby avoiding the cost associated with such further processing. Moreover, the dual-phase hot-rolled steel strip of the present invention can readily be produced employing conventional hot strip mill production practices without modification, and wherein the resultant steel strip product is characterized as having a low initial yield strength and satisfactory elongation characteristics, enabling deep drawing thereof employing conventional tooling at conventional press forces without encountering fracture or tearing of the stock during formation. The high work hardening characteristic of the steel strip product effects an increase in its yield strength during fabrication to a magnitude of about 80 ksi, enabling the use of thinner gauges and a corresponding significant reduction in the weight of the automobile components over conventional parts made from present-day moderate strength steels.